Hanset for a multiple channel communication system and use thereof

ABSTRACT

A handset (24, 25) in a radio communication system (20) determines the quality of a channel (500 and 502) in response to an RSSI signal generated by a receiver (174) in combination with either an eye closure measurement (510) or a bit error rate detected by a CRC codeword in received digital information (502). The battery operated handset is also capable of conserving scanning power while operating in an area having high channel quality (520, 560, 566). The handset also has a method for determining the amount of handovers between base stations (21, 22, 23) by use of dual channel quality thresholds (582, 590) which require a higher quality threshold for establishing communication with a base station and a reduction in channel quality before a handover to a new base station.

This is a continuation-in-part of application Ser. No. 07/823,531, filedJan. 21, 1992 now abandoned.

FIELD OF THE INVENTION

This invention relates in general to radio communications, and inparticular to channel allocation of radio devices for two-way radiocommunications.

BACKGROUND OF THE INVENTION

Radio communications involves either one-way (e.g. selective callsignalling or paging systems) or two-way communications (cellular,cordless telephone, or digital personal communication systems) overradio waves. Communication takes place on channels, comprising timedivision multiplexed (TDM) time slots or frequency division multiplexed(FDM) frequencies, or a combination thereof.

For various radio communications, fixed radio frequency spectrum areassigned. For example, in the United States, the Federal CommunicationsCommission reserves various portions of the radio frequency spectrum todesignated communication services. The radio frequency use is thereforelimited to assigned services. The radio frequency use is more overtaxedwithin certain areas where the number of frequencies allocatable forcommunicating between radio transceivers in a system are severelylimited.

As communication between radio transceivers is initiated, the systemallocates a channel or channels for communication service therebetween.With the increase in radio communications, such as cellular and cordlesstelephone usage, the method for allocating channels must allocatechannels utilizing the assigned frequencies in a manner to accommodateever increasing concurrent users. Signal interference may result frommultiple simultaneous usage of the same channel in neighboringgeographical areas (co-channel interference) or usage of adjacentchannels in the same geographical areas. The resulting interferencereduces the level of system service quality. It is therefore a primaryconcern of the system operator to allocate channels for communication ina manner to allow the greatest efficiency of usage while reducinginterference in order to maintain a certain level of service quality.

There are generally two types of channel allocation methods: fixedchannel allocation and dynamic channel allocation. Fixed channelallocation methods fix the channel assignments during the entire courseof operation. Since the channels are allocated only once, the fixedchannel allocation method can be very time intensive and, therefore,have a good chance to provide a high level of channel reuse for anygiven conditions. Fixed channel allocation methods are simple and mayapproach being optimal in terms of channel reuse to any given trafficpattern for a given system. But the fixed channel allocation is notadaptive to a changing service environment. Also, to add or remove abase station from the system is cumbersome and fixed channel allocationmethods are unable to automatically initialize the channel allocation.

Dynamic channel allocation methods, on the other hand, allocate channelsin accordance with a method which is adaptive to traffic and environmentchanges. Since dynamic channel allocation methods do not assignchannels, channels can be used in any area as needed. In addition, mostdynamic channel allocation methods can initialize automatically.Unfortunately most of the existing dynamic channel allocation methodsare too dynamic to have good performance in terms of channel reuse.

Secondarily, a portable handset for communication with the communicationsystem must also determine the quality of the available channel forcommunicating with the base stations of the communication system.However, since the base stations may be in close proximity and using thesame channels to communicate with different handsets, the determinationof channel quality by simply measuring the power of a received carrierat the handset, including averaging the received signal signal power toreduce fading effects, may produce undesirable results. The carriersignal energy produced by two adjacent base stations may apparently showa good channel quality, while in reality, the interference between thetwo base stations may result in a low quality of reception at thehandset.

Also, since the handset must be prepared to receive a call at any time,the handset must maintain its own list of available quality channels.This requires periodically scanning and determining the quality levelsof the channels by the handset in order to maintain a handset channellist. However, since the handset is portable and battery powered,periodic scanning of all of the channels in order to maintain thehandset channel list reduces the life of the battery of the handset.

It is preferable to maintain an optimally high quality channel interfacebetween the handset and a base station while a call is in progress.However, if handsets were excessively engaging in a handover processbetween changing channel quality base stations of the communicationsystem, the communication system would be overly burdened with basestation handover information. The handover information is communicatedwithin the system without charge to the user. Thus the expense isabsorbed by the system owner and thus the system owner has reducedprofitability. Furthermore, since the handover information may use thesame communications links as user call conversations, the addition ofthe handover information to these communication links reduces the callthroughput of the communication system.

Thus what is needed is a way to improve the channel quality measurementmade by the handset. Also what is needed is a method of improving thebattery life of the handset while scanning What is also needed is amethod for regulating the amount of handovers occurring within thesystem. What is further needed is a method which allows for establishinga high quality channel between the base station and the handset, whereinboth the handset and the base station make individual determinations onthe quality of the channel and the resulting selected channel is acombination of the individual determinations.

SUMMARY OF THE INVENTION

In carrying out the objects of the present invention in one form, thereis provided a portable handset transceiver for determining a qualitylevel of a communication channel having an information signal modulatedupon a radio frequency carrier for communication between the portablehandset and a base station. The handset comprises a receiver forreceiving the radio frequency carrier, and for generating a receivedsignal strength indicator for determining a received signal qualitylevel of the radio frequency carrier. The receiver further being fordemodulating the information signal. And the handset further comprisinga controller for determining an information quality level of theinformation signal, analyzing the received signal quality level and theinformation quality level, and generating a channel quality level signalin response thereto.

In another form, there is provided in a portable handset, a method ofreducing power consumption of the handset while scanning a predeterminednumber of channels. The method comprising the steps of scanning anddetermining a quality level of each of the predetermined number ofchannels within a first time interval, wherein a receiver consumes afirst amount of power during the first time interval, and generating adelay increase signal if at least one of the predetermined number ofchannels from said step (a) of scanning has a quality level above afirst predetermined level. The method further comprising the step ofscanning and determining the quality level of each of the predeterminednumber of channels within a second time interval longer than the firsttime interval in response to the delay increase signal, wherein thereceiver consumes a second amount of power less than the first amount ofpower during the second interval.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 through 14 show a first embodiment of the present invention.

FIG. 1 is a diagram of a cordless telephone communication system inaccordance with the present invention.

FIG. 2 is a diagram of a service area of a multiple base stationcordless telephone communication system in accordance with the presentinvention.

FIG. 3 is a block diagram of a base station in accordance with thepresent invention.

FIG. 4 is a flowchart of the operation of the controller of the basestation of FIG. 3 in accordance with the preferred embodiment of thepresent invention.

FIG. 5 is a flowchart of the operation of the channel allocator of thebase station of FIG. 3 in accordance with the preferred embodiment ofthe present invention.

FIG. 6 is a block diagram of a cordless telephone handset in accordancewith the preferred embodiment of the present invention.

FIGS. 7, 8, 9 are flowcharts of the operation of the controller of thehandset FIG. 6 in accordance with the preferred embodiment of thepresent invention.

FIG. 10 is a flowchart of the operation of the call initiation routineof the controller of the handset FIG. 6 in accordance with an alternateembodiment of the present invention.

FIGS. 11, 12, 13, and 14 are flowcharts of the operation of the channelallocator of the base station of FIG. 3 in accordance with a secondembodiment of the present invention.

FIGS. 15 through 20 show various aspects of a second embodiment for thehandset of FIG. 6.

FIG. 15 shows a channel matrix of the dynamic channel assignment systemoperating in accordance with the preferred embodiment of the presentinvention.

FIG. 16 shows an idealized eye pattern of a two level FM signaloperating in accordance with the preferred embodiment of the presentinvention.

FIG. 17 shows flow chart of a first method for determining channelquality.

FIG. 18 shows flow chart of a second method for determining signalquality.

FIG. 19 shows a flowchart for reducing power consumption while formaintaining a handset channel list.

FIG. 20 shows a flow chart of a handset initiating a call andmaintaining the call during a handover environment.

DETAILED DESCRIPTION OF THE INVENTION

I. A First Embodiment of the Communication System

The present invention is applicable to all forms of radio communication,such as cellular radio telephone systems or cordless telephone systems,which have a need to allocate communication channels to users. Thepreferred embodiment of the present invention is described herein inreference to a cordless telephone system operating in accordance withthe Digital European Cordless Telephone (DECT) specification.

Referring to FIG. 1, a DECT system 20 comprises telepoint base stations,such as 21, 22, or 23, designed for inbound or outbound calling to orfrom handsets, such as 24 or 25. The base stations 21, 22, or 23 arecoupled to a node 26 for coupling base stations into a cluster forcovering a localized area such as an office building or a shopping mall.The nodes 26, 26' may be coupled to a central network control 28 foroperation of a radio communication system. The nodes 26, 26' are coupledto the public switched telephone network 30 through which calls arerouted to or from a conventional telephone 32 via connections 30a or 30bestablished through the public switched telephone network 30. When acaller calling from a handset 24 or 25, wants to place a call, a radiocommunications link is formed with a base station 21, 22, or 23 and thecall is connected to the conventional telephone 32 via the node 26 andthe connection 30a. When a caller, calling from a conventional telephone32, wishes to reach a particular handset subscriber, the caller places acall through a connection 30a established through the public switchedtelephone network 30 to a node 26. The node 26 instruct the basestations 21, 22, and 23 to page for the handset. The handset responds tothe page by signalling the base station 21 in order to couple thehandset 24 to the telephone 32.

Channel allocation, whether the call is outbound from the cordlesstelephone handset or inbound thereto, is handled by each base station21, 22, or 23. A handset 24 attempts to establish communication with thebase station 21, 22, or 23 having the strongest signal at the handset24. Once communication is initiated between the handset and a basestation, the base station 21, 22, or 23 allocates at least onecommunication channel to the handset 23, 24, or 25. The channel isallocated in accordance with the preferred embodiment of the presentinvention by selecting a channel from a Preferred Channel List (PCL) asdescribed below. The PCL is maintained at each base station 21, 22, or23, independently, allowing fully distributed channel allocationthroughout the system, therefore not overburdening a node 26 or thecentral control 28.

Referring to FIG. 2, each base station 21a-c, 22a-b, and 23a-b, has anassociated coverage area 40a-c, 41a-b, and 42a-b, respectively. Forexample, base station 21b will provide communications for any handsetwithin coverage area 40b. The coverage area provided by a base stationmay vary based upon materials forming the environment in the coveragearea. In cordless telephone systems, the base stations are preferablypositioned to form coverage areas such that a cluster of base stationscover a designated area such as a shopping mall, an airport, an officecomplex, or a designated geographic area. When allocating channels foruse and assigning channels to the handsets as requested, the basestation 21b considers the history of channel quality for channels usedin the area 40b, as described below.

Referring to FIG. 3, a block diagram of a base station 21 in accordancewith the present invention is shown. Radio communication with the basestation is provided by an antenna 54 and a conventional communicationstransceiver 56. The communications transceiver is coupled to a callhandler block 58 of a controller 60. The call handler block 58, inaccordance with the present invention, receives the call initiationrequests, transmits the allocation of communication channels tohandsets, and maintains the channel for communication with the handsetuntil the service or call is disconnected.

The controller 60 controls the operation of the base station 21 for alloperations. The controller 60, in accordance with the present invention,also comprises a channel allocator block 62 for allocating channels. Thecontroller 60 and the channel allocator 62 of the preferred embodimentof the present invention are coupled to a memory 64 for accessing andmaintaining information on the system's channels. In accordance with thepreferred embodiment of the present invention, a Preferred Channel List(PCL) is stored therein. The PCL is a list developed from all of thechannels available to the system. Each base station typically uses onlya subset of the channels available to the system. The controller 60maintains the PCL by measuring and recording a history of channelquality measurements as explained below.

In accordance with an alternative embodiment of the present invention,an assigned channel list and a borrowed channel candidate list arestored in the memory 64. The controller 60 is coupled to the node 26 andmay allow communication with the node 26 or the network center 28(FIG. 1) via a network connection 68 for maintenance of the assignedchannel list and the borrowed channel candidate list stored in thememory 64.

Referring to FIGS. 4 and 5, the channel allocation method of thepreferred embodiment of the present invention is described. The fullydistributed dynamic channel allocation method of the present inventioncombines the advantages of the fixed and dynamic channel allocationmethods. For example, the method of the present invention has channelallocation initialization and is adaptive to a slowly changingenvironment like the dynamic channel allocation methods. In addition, inheavy traffic the distributed dynamic channel allocation method of thepresent invention simulates an optimal fixed channel allocation methodin the sense that the method provides an improved compact layout ofchannel reuse. Since all the decisions are based on local measurementsat the base station, the channel allocation method of the preferredembodiment of the present invention is simple and fully distributed inthe sense that no direct connection between base stations is necessary.This allows for independent operation by each base station and does notburden the nodes 26 or the central network control 28 (FIG. 1) forchannel allocation.

The measurements made by the base stations concern the quality of achannel in the system. The quality is described by a function ƒdescribing successful channel service, where a channel service isdefined to be successful if it is neither blocked nor interrupted. Forexample,

    ƒ (p.sub.s,Margin, cq)

where

p_(s) is the probability of success or P(success); Margin isEquality>Threshold(^(quality-Threshold)), that is, Margin is the meanvalue of (quality-Threshold under the condition of quality>Threshold,where quality is any measurement defined by the system;

Threshold is the lower bound of the quality for a channel to beconsidered as a good channel;

and cq is the current channel quality.

The requirements for the quality function are that it should be anincreasing function of p_(s) and cq and a decreasing function of Margin.The definition of the quality function for a channel (ƒ) in accordancewith the preferred embodiment of the present invention is: ##EQU1##where L, M, N: weights defined by the system, for example L=4.0, M=10,and N=1.0;

N_(s) : number of successful calls;

N_(b) : number of blocked calls; and

N_(i) : number of interrupted calls.

The method of channel allocation of the preferred embodiment of thepresent invention allocates channels with a combination of a highprobability of success and a low probability of being blocked orinterrupted. This is accomplished by defining p_(s) to include not onlythe effects of the number of successfully completed calls and number ofblocked calls but also the effect of the number of interrupted callswith an assigned weight M. Preferably M is set equal to 10, therebyweighting the occurrence of interrupted calls ten to one over blocked orsuccessful calls. In addition, the method of the preferred embodiment ofthe present invention advantageously allocates channels with a lowmargin and a high channel quality. The lower margin preferably allocateschannels not too good, that is, channels usually operating at thequality approaching the threshold from above.

In accordance with the preferred embodiment of the present invention,each base station in the system maintains a Preferred Channel List(PCL). The channels are listed in a decreasing sequence of the values oftheir quality function. The channel quality of the channels for a basestation will be updated whenever any of the following three recordableevents happens: (a) a service (e.g., a call) is finished successfully;(b) a service is interrupted; or (c) a channel is rejected for serviceinitiation due to bad quality (i.e. blocked). The PCL is maintained inmemory 64 (FIG. 3) of each base station 21. The controller 60 (FIG. 3)of the base station updates the Preferred Channel List (PCL) while thechannel allocator 62 (FIG. 3) utilizes the PCL for allocating channelsin response to service initiation requests.

Referring to FIG. 4, the PCL updating process of the base stationcontroller 60 in accordance with the preferred embodiment of the presentinvention is shown. When a base station is first installed in the system100 it assigns an initial probability p_(s0) greater than zero to p_(s)(i.e., p_(s) =p_(s0) >0) 102 and the Margins are initialized 103. Theinitial probability of success (p_(s0)) is a parameter that affects thegrade of service of a base station when the base station is in itsinitialization stage. If the p_(s0) is assigned a small value theun-used channel will not be used unless the grade of service of the usedchannel degrades a great amount (i.e., dropping lower than p_(s0)). Theassignment of the initial value of p_(s) =p_(s0) >0 makes the value ofthe quality function of the used channel and that of the un-usedchannels comparable. From the definition of the quality function we seethat the quality function is only meaningful for used channels having aquality function ƒ(p_(s),Margin, cq)>0. By assigning p_(s) =p_(s0) >0,for example 0.5, an un-used assigned channel will be considered forallocation before a used channel only if the un-used channel has ahigher quality value than that of the used channel. In case the initialvalue of the channel is assigned p_(s) =p_(s0) =0 the un-used channelswould never be considered because the quality value of the used channelwould always be higher than that of the un-used channels. The Marginsare also initialized 103 to equal values. The initial PCL is then formedby randomly positioning the channels of the system and stored 104.

Processing then awaits the occurrence of one of the three recordableevents on a channel: (a) a call on the channel is finished successfully106; (b) an initiation request on the channel is blocked 118; or (c) acall on the channel is interrupted 122. The channel allocator 62 signalsthe controller 60 that a block event has occurred 118 when a callinitiation request is blocked. The call handler 58 follows the progressof a call on a channel and signals the controller 60 that a succeedevent has occurred 106 upon successful completion of the call. And thecall handler 58 signals the controller 60 that an interrupt event hasoccurred 122 when the call handler 58 is signalled by the handset thatthe call is interrupted.

If a succeed event on a channel has occurred (i.e., a call on thechannel is finished successfully) 106, the success event counter, N_(s),for that channel is incremented by one 108 and the Margin for thatchannel is updated 110. Previous proposals for using a channel with ameasured Margin use an instantaneous value for the Margin which is onlyupdated when needed, i.e., at the call setup stage. The method of thepresent invention, though, updates the Margin 110 after each successfulcompletion of a call 106, thereby utilizing a more meaningful Marginbased on the history of the channel. The Margin being updated after eachsuccessful call realizes a mean Margin value which advantageouslyimproves the concept of Margin over the prior instantaneous marginmeasurement. Also, the margin value after a successful call is a truermeasurement of channel Margin than the margin measured at callconnection setup.

After updating the Margin 110, the probability of success, p_(s), forthe channel is then updated 112 and the quality function, ƒ, for thechannel is updated 114. The Preferred Channel List (PCL) is adjusted 116in accordance with the updated quality function, ƒ, value. For example,the success event could increase the quality function, for the channeland may allow the channel to move to a higher position in the PCL.

After adjusting the PCL 116, processing returns to await occurrence ofone of the three recordable events 106, 118, or 122. If a block event ona channel has occurred 118, the block event counter N_(b) for thatchannel is incremented by one 120. The probability of success p_(s) forthe channel is then updated 112, the quality function ƒ for the channelis updated 114, and the PCL is adjusted 116 in accordance with theupdated quality function value. Likewise, if an interrupt event on achannel has occurred 122, the interrupt event counter N_(i) for thatchannel is incremented by one 124, the probability of success (p_(s))for the channel is updated 112, the quality function (ƒ) for the channelis updated 114, and the PCL is adjusted 116. After each adjustment ofthe PCL 116, processing returns to await occurrence of one of the threerecordable events 106, 118, or 122.

In this manner, a current Preferred Channel List (PCL) is maintainedwherein the channel with the highest value of the quality functionmeasured on past history is at the top of the PCL. The other channelsare positioned in the PCL at ever decreasing values of the qualityfunction measured. As stated above, the channel allocation method of thepreferred embodiment of the present invention is fully distributed inthat each base station stores a PCL in its memory 64 (FIG. 3) and thecontroller 60 maintains, or updates, the PCL as events occur.

Referring to FIG. 5, the operation of the channel allocator 62 of thebase station 21 of FIG. 3 in accordance with the preferred embodiment ofthe present invention is shown. The method for channel allocation of thepresent invention attempts to assign the channel with the best quality(i.e., the channel with the highest value of the quality function)whenever possible. The allocated channel would be the first channel inthe Preferred Channel List (PCL) that is in a free time slot (in thecase of one transceiver per base station) and currently in good quality.Alternatively, channel allocator 62 could allocate a number of channelsin good quality as a list of channels is decreasing preference to thebase station (an allocated channel list, ACL), from which the handsetwould choose the first acceptable channel and identify to the basestation the acceptable channel.

After the base station is put into service 125, the channel allocator 62awaits a request for a channel assignment 126. When a channel assignmentrequest has been received 126, the PCL is examined to determine if thenext free (e.g., first) channel on the PCL is available 128. If thereare no free channels available for allocation 128, the channel allocator62 signals the controller 60 to inform the handset that no channel isavailable for allocation 130 and processing awaits the next channelassignment request.

If a free channel on the PCL is available for allocation 128, it isdetermined whether the time slot for the channel is busy 132 (assumingthere is one transceiver 56 per base station (FIG. 3)). If the channel'stime slot is busy 132, the PCL is examined to determine if another freechannel on the PCL is available for allocation 128. If the channel'stime slot is not busy 132, the current channel quality cq is measured134 and it is determined from the channel quality measurement whetherthe channel is now a good channel 136. Though the channel may occupy ahigh position in the PCL because it had previously been measured as agood channel, the channel may not now be a good channel. If the channelis not now a good channel 136, the channel allocator 62 signals thecontroller 60 that a block event for the channel has occurred 138 andthe next free channel on the PCL is examined to determine if it isavailable 128. If, on the other hand, the channel is measured as a goodchannel 136, the channel is assigned for communication 140. As mentionedabove, the channel allocator could perform a number of iterations ofmeasuring channels to derive a predetermined number of good channels(such as four) and inform the handset of the good channels in decreasingpreference (the ACL) 140. Processing awaits a reply from the handset fora predetermined time, M seconds 141. If a positive reply from thehandset is received identifying an acceptable channel within M seconds,the channel accepted by the handset is utilized for communication andthe channel allocator 62 signals the controller 60 that a channelassignment (i.e., a channel initiation request) has succeeded 142.Processing returns to await the next channel assignment request 126. Ifa positive reply is not received within M seconds 141, processingreturns to step 126.

In this manner, the PCL is utilized to allocate channels forcommunication. The method of channel allocation of the present inventionutilizes the mean Margin as a parameter of the quality for a channel.This parameter makes possible optimizing channel reuse in a distributedfashion. More particularly, taking the mean Margin as a parameter of thequality function, a channel with a lower mean Margin has a higherpriority over channels with higher mean Margins, leading to betterchannel reuse. The improved channel reuse is a result of therelationship that the smaller the mean Margin, the more compact thelayout of the co-channels will be and, consequently, the higher thechannel reuse. Previous proposals for using a channel with a smallMargin use an instantaneous value for Margin which is only measured atthe call initiation stage. Due to shadowing and fading effects and itsvague meaning, an instantaneous value for Margin is almost meaningless.The method of the present invention, though, uses a mean Margin which isupdated after each successful completion of a call, thereby utilizing amore meaningful mean Margin based on the history of the channel.

Referring to FIG. 6, the handset 24 of the DECT system 20 (FIG. 1)comprises an antenna 170 coupled to a transmitter circuit 172 and areceiver circuit 174. The microprocessor controller 178 receives asignal from the receiver circuit 174 indicating the received signalstrength (the RSSI signal).

A time division duplexer 176 controls the signal provided to thetransmitter 172 and received from the receiver 174 to facilitate two-waycommunications by synchronizing communications to the time slotallocated for communications. The operation of the timed divisionduplexer 176 is controlled by a signal from the microprocessorcontroller 178. The microprocessor controller 178 provides a signal to afrequency synthesizer 180 for controlling the operation thereof. Thefrequency synthesizer 180 supplies the operating frequency informationto the transmitter 172 and the receiver 174 for modulation anddemodulation of the communication signal. The controller 178 is alsocoupled to a memory 179 for accessing and updating stored information.

The signal received by the receiver 174 or transmitted by thetransmitter circuit 172 is a digitally encoded signal which passesthrough a codec 184 for digital-to-analog or analog-to-digitalconversion. The signal received via the receiver circuit 174 andconverted by the codec 184 is supplied as an analog signal to audiocircuitry 186 and thence to a speaker 188. Likewise, an analog signalreceived from a microphone 190 passes through the audio circuitry 186and is converted to a digital signal by the codec 184 before beingprovided to the transmitter circuit 172. In addition, control signals,such as call initiation requests and call disconnect requests, can beprovided from the controller 178 to the transmitter 172 for transmissiontherefrom. Control signals received by the receiver 174 are likewiseprovided to the controller 178.

For other operations, such as dialling up a telephone number, usercontrols 183 provide appropriate signals to the microprocessorcontroller 178. In addition, the microprocessor controller 178 suppliesa signal to a display driver 192 for generation of a visual message forpresentation to the user on a display 194.

Referring to FIGS. 7, 8, and 9, a flow chart of the operation of thecontroller 178 of the handset 24 (FIG. 6) is shown. Referring to FIG. 7,the process of the controller 178 (FIG. 4) starts 200 with the power upof the handset. The controller can perform three functions whileinterfacing with a base station: initiate a call 202 between the handsetand the base station, finish a call 204, and monitor a call 206. If thecontroller 178 is requested to initiate 202 a call, a call initiationroutine 210 is performed, after which processing returns to await thenext calling task request 202, 204, 206. If call finishing 204 isdetected, the handset sends a disconnect request 212 to the base stationafter which processing returns to the idle loop to await one of thecalling task requests. Finally, if call monitoring 206 is detected, acall monitoring routine 214 is performed, after which processingproceeds to await the next calling task request 202, 204, 206. If noneof the calling task requests are received, processing remains in an idleloop 202, 204, 206 awaiting one of the calling task requests.

Referring to FIG. 8, before the call initiation routine 210 begins, thehandset locks to a base station and learns the Preferred Channel List(PCL) of that base station. The handset could lock to the base stationby taking a quality measurement of a channel broadcast by the basestation, such as measuring the RSSI value (proportional to the power ofthe signal broadcast by the base station) or calculating a signal tonoise and interference ratio (S/I). The Preferred Channel List (PCL) ofthe base station is downloaded into the memory 179 (FIG. 6). The handsetmonitors the environment by regularly scanning channels to lock to thestrongest base station. The base station broadcasts its PCL on itschannel regularly or on demand. Thus, the handset is locked to a basestation and knows the PCL of the base station before call initiation210.

First, the Preferred Channel List (PCL) of the selected base station isexamined to determine if the next (e.g., first) channel in the PCL isavailable 222. If a channel is available from the PCL 222, the qualityof the channel is measured to determine if the channel quality is abovea threshold Th 224. If the channel quality is not above the threshold Th224, the next channel in the PCL is selected, if available 222. If thechannel quality is above the threshold Th 224, a connection request issent to the base station using the channel 226. If a positive reply isnot received from the base station within a predetermined time period Ti228, the next channel in the PCL is selected, if available 222. If apositive reply is received from the base station within Ti seconds 228,the channel quality of the allocated channel is measured to determine ifit is a good channel 229. If the allocated channel is a good channel229, a communication connection (i.e., radio link) is setup between thehandset and the base station using the channel 230 and processingreturns 232 to await the next calling task request 202, 204, 206.

If the allocated channel is not good 229, processing awaits reception ofa positive reply from the base station for Ti+d seconds 234 indicatingan additional allocated channel. If a positive reply is received withinTi+d seconds 234, the channel quality of the additional allocatedchannel is measured to determine if it is a good channel 229. If apositive reply is not received within Ti+d seconds 234, the user isinformed that the channel connection request failed 236 (i.e., nochannel is available for allocation from the selected base station) andprocessing returns 232 to await the next calling task request 202, 204,206. Also, when there are no channels available for allocation 222, theuser is informed that the channel connection request failed 236 andprocessing returns 232 to await the next calling task request 202, 204,206.

Referring next to FIG. 9, the call monitoring routine 214 is shown.While a call is in progress 214, the channel quality is monitored by thecontroller 178 (FIG. 6) to determine whether the channel quality hasbecome poor for continued use 250. The channel quality is poor when thequality falls below a threshold Th 250. When it is determined that thechannel quality has fallen below the threshold 250, the call initiationroutine 210, as described above, is accessed. If a call is notsuccessfully initiated within a predetermined time 252, the user isnotified that the call is suspended 254 and a call interrupt notice issent to the the base station 256 for maintaining the Preferred ChannelList, as described above. Processing returns 257 to the idle loop 202,204, 206 (FIG. 7) to await one of the calling task requests.

If a call is successfully initiated on another channel 252, a "handover"is performed, handing the call over from one channel to another. A newconnection is established between a base station and the handset on thenew channel 258. A disconnect request is sent on the old channel 259 toremove the old connection. Processing then returns 257 to the idle loop202, 204, 206 (FIG. 7) to await one of the calling task requests.

The preferred embodiment of the present invention is an apparatus andmethod for fully distributed dynamic channel allocation. An alternatemethod for channel allocation utilizing the present invention is anoptimal dynamic channel allocation. The alternate method of channelallocation described herein combines fixed channel allocation anddynamic channel allocation and optimizes channel reuse by keeping thereuse distances (i.e., the distances between co-channel base stations)as small as possible with respect to the changing environment. Thechannel allocation method allows channel borrowing functions under lighttraffic. In heavy traffic, the alternate method described hereinbelowapproaches an optimal fixed channel allocation method with a compactlayout of channel reuse in respect to a slowly changing environment. Inthe channel allocation adjustment process described herein, a meaningfulmeasurement of the channel quality is the S/I measured when all thefirst tier co-channels are busy. The concept of locally heavy loadedperiods is introduced to allow meaningful measurement of the channelquality. The locally heavy loaded period of channel i is detected eitherby local measurement of the environment (a distributed version) or byreceiving messages from a center (a central control version) such as thenetwork center 28 (FIG. 1).

At any one time, a subset of the channels available to the system areused by a base station. These channels are "assigned" to the basestation and the base station is the "home site" of these channels. Thecriteria for improved channel allocation in accordance with the methodof the present invention is that channel i will be "assigned" to site jif channel i offers guaranteed quality of service to site j unless someother site borrows it, and site j has the most efficient usage ofchannel i at the particular time period.

In the alternate embodiment, the quality of a channel is measured by thesignal to noise plus interference ratio (S/I). The minimum requiredquality for the channel to be used is defined by the threshold--Th. Achannel is found to be in poor quality if S/I<Th. A channel is perfectif 0≦S/I-Th≦ε where ε is a given constant which defines the sensitivityof the channel allocation method. A channel is found in too good qualityif S/I>>Th.

Making channel allocation decisions based on the measurement of S/I inaccordance with the present invention results in advantages over systemsthat make decisions based on a deductive method. For example, the methodof the present invention is simple, accurate, and effective. Though S/Imeasurement is simple, the S/I may have a vague meaning: e.g., if S/I istoo good the reason may be either all of the co-channels are not busy orall of the co-channels are too far away; or if the S/I is poor thereason may be some other site is borrowing the channel or someco-channel sites are too close. The vague meaning of S/I indicates thata channel allocation method based solely on a single measurement of S/Iwill lead to poor performance. The channel allocation method of thepresent invention, therefore, is concerned with a worst case S/Imeasurement. The worst case S/I of channel i, SI_(w) (i), occurs whenall the main co-channels (first tier) of channel i are in use. SI_(w)(i) can be derived from m independent measurements (m≧1), where theprobability, p_(s) /I, that the worst case S/I cannot be found in mindependent measurements is a function of the mean idle-to-busy ratio:##EQU2## where n is the number of co-channels; and

r=max_(k) (E_(k) (I_(i))/E_(k) (B_(i))), where k designates theco-channel base stations.

The present invention advantageously utilizes the measurement S/I withthe concept of locally light or locally heavy loaded periods tooptimally allocate channels. A channel i at site j can be said to be ina locally heavy loaded period if all the important interference, theprime co-channels (which are defined for simplicity as the first tierco-channels) are busy. When not locally heavy loaded, a channel isconsidered locally lightly loaded.

If a measurement S/I is too good in locally heavy loaded period we arestill not quite sure if the quality of the channel is really too good.The problem is solved by taking m independent measurements during thelocally heavy loaded period. The quality of the channel is described bythe worst signal-to-noise ratio of channel i, SI_(w) (i), among the mmeasurements, which greatly increases the creditability of themeasurement.

    SI.sub.w (i)=min.sub.l L (S.sub.l (i)/I.sub.l (i))

where L is a set containing the last m measurements measured after thechannel is found to be in a locally heavy loaded period.

Channel borrowing is allowed in locally light loaded periods, but is notallowed in locally heavy loaded periods. By allowing channel borrowingonly in locally light loaded periods and the channel allocationadjustment process only in locally heavy loaded period, the vagueness ofthe meaning of S/I is removed. For example: if S/I is found in poorquality in a locally light loaded period, the measurement means that anadjacent base station is borrowing the channel (co-channel interferencein a light loaded period is not a concern). If S/I is found poor in alocally heavy loaded period, the measurement means that some co-channelsite is too close since channel borrowing in accordance with the presentinvention is not permitted in locally heavy loaded periods.

Each base station in accordance with the alternate embodiment of thepresent invention performs a channel assignment adjustment whereby achannel is removed from a base station's list of assigned channels.Whenever, in the channel assignment adjustment process, a channel in alocally heavy loaded period is found to be poor, a replacement "homesite" for the channel may be found. The new "home site" should have abetter quality (S/I≧Th) or a more efficient use of the channel. Achannel is poor if SI_(w) (i) is less than the threshold, Th, forchannel quality. The channel is too good when SI_(w) -Th>ε.

In accordance with the method for optimal dynamic channel allocation,channels are re-assigned through competition. The base station which hasbetter usage of the channel will eventually have that channel assignedthereto. The key point of the strategy is the introduction of a waitingtime Ti for monitoring channel i where

    T.sub.i =T.sub.s +t.sub.i

where T_(s) is a constant defined to be long enough so that the basestation can discover if the channel i is in a locally heavy period andthe quality of channel i can be properly measured, and t_(i) is ameasurement of the margin and importance of channel i to the associatedbase station j defined as ##EQU3## where traffic(j) is the currenttraffic at site j.

The home base stations of channel i with too good quality of the channelare required to stop using channel i for at least T_(i) seconds so thatthe other base stations may have the opportunity to make a measurementof the channel. The base station who has the best usage of the channel(i.e. has the smallest t_(i)) will seize the channel and become the newhome base station.

In accordance with the alternate embodiment of the present invention,the handset 24 maintains a table in the memory 179 (FIG. 6) of basestations within the system to allow the controller 178 to find availablechannels as described below. The base station table contains a list ofpreferred sites organized in descending order of P(j)≧P_(t), as measuredby the controller 178 from the RSSI values of the base station'schannels, where P(j) is the measured signal strength from base station jand P_(t) is the acceptable threshold of signal strength. There may beonly one entry in the table, or even no entries.

Referring to FIG. 10, a flow chart of the operation of the controller178 of the handset 24 (FIG. 6) when setting up communication with a basestation call handler 58 (FIG. 3) in accordance with the presentinvention is shown. The call handler 58 interfaces the controller 60(FIG. 3) of the base station 21 with the handset 24. The operation ofthe controller 178 of the handset 24 (FIG. 6) in accordance with thealternate embodiment of the present invention operates as describedabove in reference to FIGS. 7 and 9. A "handover" during the callmonitoring routine (FIG. 9) could alternatively be initiated if thechannel quality, as measured by S/I becomes poor (i.e., S/I<Th). Thecall initiation routine 210', though, differs, as depicted in FIG. 10.The call initiation routine 210' begins by initializing an index n(j)for every base station in the base station table stored in the memory179 (FIG. 6) to one 260. When the index n(j) of a base station j equalsone, processing is examining the base station j for the first time; whenthe index n(j) equals two, the channel of the base station j havepreviously been examined.

After initialization of the indexes n(j) 260, the controller 178examines the table in the memory 179. If there is a base station j witha channel having a signal power P(j) greater than or equal to thethreshold signal power P_(t) and the index n(j) equals one 262, aconnection request is sent to the first such base station in the basestation table 264, the connection request indicating that a variable xshould be set to zero. When the variable x is set to zero, the basestation attempts to allocate one of its assigned channels to thehandset. If the variable x is set to one, the base station attempts toallocate one of its borrowed channels to the handset if no good enoughassigned channel exists.

After the connection request is sent 264, the index n(j) for the basestation j is set equal to two 266 and processing awaits reception of areply 268. If the reply to the connection request is received within apredetermined time and is positive 268, the call is initiated 270 on theallocated channel and processing returns 272 to the idle loop 202, 204,206 (FIG. 7) to await one of the calling task requests. A positive replymeans there is at least one channel allocated in the reply that isacceptable for the handset. If the reply to the connection request isnot positive or a reply is not received within the predetermined time268, processing returns to step 262.

If there are no base stations j having P(j) greater than or equal toP_(t) and an index n(j) equal to one 262, then the table stored in thememory 179 (FIG. 6) is examined to determine if there are any basestations j having P(j) greater than or equal to P_(t) and an index n(j)equal to two 274. A connection request is then sent to the first suchbase station in the base station table 276, the connection requestindicating that the variable x should be set to one. The index n(j) forthe base station j is then set equal to three 278 and processing awaitsreception of a reply 280. If the reply to the connection request ispositive and received within a predetermined time 280, the call isinitiated 270 on the allocated channel and processing returns 272 to theidle loop 202, 204, 206 (FIG. 7) to await one of the calling taskrequests. If the reply to the connection request is not positive or areply is not received within the predetermined time 280, processingreturns to step 274. In other words, after all of the base stations inthe table (base stations with P less than P_(t) are not included in thetable) stored in the memory 179 have been sent a connection request onceto allocate an assigned channel and no assigned channel is acceptable,the list is gone through again to send a connection request to thelisted base stations to assign a borrowed channel.

If there are no base stations j having P(j) greater than or equal toP_(t) and an index n(j) equal to two 274, the handset user is informedthat the call initiation has failed 282, and processing returns 272 tothe idle loop 202, 204, 206 (FIG. 7) to await one of the calling taskrequests. Call initiation failure occurs when either (a) there are nobase stations with a channel power P greater than or equal to thethreshold P_(t) (i.e., the handset is out of range of any basestations), or (b) a positive reply has not been received from connectionrequests within the predetermined time to the base stations in the tablefor allocation of a channel assigned (x=0) to the base station orborrowed (x=1) by the base station. A call is established if at leastone of the base stations in the list stored in the memory 179 has a freechannel (if no assigned channels are available, borrowed channels areattempted to be allocated) with S/I≧Th as measured at both the basestations, as described below and the handsets. Otherwise the call isblocked.

Referring to FIGS. 11, 12, 13, and 14, the optimal dynamic channelallocation routine is described in accordance with the alternateembodiment of the present invention. The operation of the channelallocator 62 (FIG. 3) starts 300 with the startup of the base station.The channel allocator 62 determines whether a disconnect request hasbeen received 302. If the channel allocator 62 receives a disconnectrequest 302, a disconnect routine 304 is performed, after whichprocessing moves into an environment monitoring routine 306. If aconnection request is received 308, a connect routine 310 is performed.Processing then returns to the environment monitoring routine 306.Otherwise, processing monitors the channel environment via theenvironment monitoring routine 306. Alternatively, a more efficientmethod for performing the channel allocation would be to implement theenvironment monitoring routine with interrupts for activation of thedisconnect routine or the connect routine when a disconnect requestinterrupt or a connection request interrupt are received, respectively.

Referring to FIG. 12, the disconnect routine 304 first determines if thecall to be disconnected was on an assigned channel 320. If the call wasnot on an assigned channel 320 (i.e., the call was on a borrowedchannel), the call is disconnected 321 and processing returns 322 tomonitor the environment 306 (FIG. 11).

If the call was on an assigned channel 320, it is determined whether aborrowed channel is in use 324. If no borrowed channels are in use, thecall is disconnected 321 and processing returns 322 to monitor theenvironment 306 (FIG. 11). If a borrowed channel is in use 324, theassigned channel is examined to determine if the assigned channel isacceptable to the borrowed channel traffic 325. If the assigned channelis not acceptable to the borrowed channel traffic 325, the call isdisconnected 321 and processing returns 322 to monitor the environment306 (FIG. 11).

If the assigned channel is acceptable 325, the assigned channel is freed(i.e., the call on the assigned channel is disconnected) 326 and a newcall is initiated on the assigned channel 327. In accordance with thepresent invention, a base station advantageously attempts to return aborrowed channel to its home base station as soon as possible. Thusafter a call is finished on an assigned channel, the traffic on aborrowed channel will be transferred to the finished call channel ifacceptable and the borrowed channel will be returned to its home basestation. By returning the borrowed channels to their home base stationsexpeditiously, the present invention advantageously attempts to maintainan ideal environment among the base stations Which lend or borrowchannels, and the communication traffic utilizes the assigned channelsas far as possible. In accordance therewith, the traffic on one of theborrowed channels in use is transferred 328 to the newly freed assignedchannel and the borrowed channel is freed 330. Processing then returns322 to monitor the environment 306 (FIG. 11).

Referring to FIG. 13, the connect routine 310 begins by examining thechannels assigned to the base station to determine if there is at leastone free assigned channel in good quality 340. The assigned channels ofa base station are always the first choices of traffic. If there is nofree assigned channel in good quality 340, the value of x transmitted bythe requesting handset is examined to see if x equals one 342. If x doesnot equal one 342, the handset is requesting an assigned channel of thebase station. Thus, if there is no free assigned channels 340 and x doesnot equal one 342, the requesting handset is signalled that a channel isnot available (i.e., a negative reply is sent to the handset requestingconnection) 344. Processing then returns 346.

If x is equal to one 342, the channels are checked to see if there is achannel to be borrowed 348. A channel can be borrowed if the channel isnot "assigned" to the base station but nevertheless has good measuredchannel quality at the base station and is not in a locally heavy loadedperiod. If there are no channels to be borrowed 348, the requestinghandset is signalled that a channel is not available 344 and processingreturns 346. If there is a channel to be borrowed 348, the borrowedchannel is allocated for use by the requesting handset 350 and therequesting handset is signalled that a channel is available (i.e., areply is sent to the handset requesting connection) and the borrowedchannel is identified 352. Processing then returns 346 to monitor theenvironment 306 (FIG. 11).

If there is a free assigned channel 340, processing determines whetherthe base station is lightly loaded 354. If the base station is notlightly loaded 354, a free assigned channel in good quality is allocated356 for use by the requesting handset and the handset is signalled thata channel is available, the allocated channel being identified thereto358. Processing then returns 346. If the base station is lightly loaded354, the channel allocator 62 (FIG. 3) attempts to use only a portion ofthe assigned channels by allocating a free channel in good quality froma designated portion of the assigned channels 360 (for example, thelower indexed channels). In this manner, a lightly loaded base stationwill try to use a portion of the assigned channels with S/I≧Th so that abusy adjacent base station can borrow the un-used channels. Yet anassigned channel should not be used too much. An un-used assignedchannel i will not be used until at least one of the used assignedchannels is being used too much. A channel is determined to be used toomuch if the spacing between successive busy period of the channel,T_(spac), is less than τ. In other words, in light traffic the channelallocation method of the present invention tries to use a minimum numberof channels assigned to it with all the used channels having S/I≧Th andT_(spac) ≧τ.

The requesting handset is thereafter signalled that a channel (withinthe designated portion, if possible) is available, the allocated channelbeing identified thereto 358 and processing returning 346. As describedabove, the channel allocator 62 (FIG. 3) could alternatively allocate alist of available channels in decreasing preference from which thehandset can choose an acceptable channel.

Referring to FIG. 14, the environment monitoring routine 306sequentially monitors the channels of the radio communication system oneach pass through the loop 302, 308, 306 (FIG. 11). In order tosequentially monitor the channels, the environment monitoring routine306 initially increments a counter i to examine the next channel i 362.If channel i is an assigned channel of the base station 364, examinationof channel i at the base station determines whether channel i is locallyheavy loaded 366. As described above, a channel i is found to be in alocally heavy loaded period at a base station j if all the importantinterferences (i.e. first tier co-channel interferences) are busy.

If channel i is locally heavy loaded 366, a channel reassignment processtakes place. The channel reassignment process adapts to the slowlychanging environment and is realized by a channel assignment adjustmentoperation. The channel assignment adjustment operation will not bestarted unless the channel i is found in a locally heavy loaded periodat the base station 66. If the channel is found not in a locally heavyloaded period any more 366, the channel assignment adjustment operationwill stop and processing will return 368 to the loop 302, 308, 306 (FIG.11).

The channel assignment adjustment operation first examines the qualityof the channel 370. Site j will release channel i if the quality of thechannel becomes poor (SI_(w) <Th) 370 by removing the channel i from thebase station's assigned channel list 372. Processing then returns 368 tothe loop 302, 308, 306 (FIG. 11).

If the quality of the channel is not poor 370, but is determined to betoo good (i.e., SI_(w) -Th>ε) 374, the base station tries to releasechannel i so that another base station which has more efficient usage ofchannel i may be the new "home site" of channel i. First, the time sincechannel i was last tested is examined 376 to see if it is greater than apredetermined minimum time between channel assignment adjustments, Tseconds. If T seconds has transpired 376 since the last assignmentadjustment of channel i, use of channel i is terminated for T_(i)seconds 378 to continuously monitor channel i for channel quality. Asdescribed above, T_(i) =T_(s) +t_(i) where T_(s) is a constant and t_(i)is a measurement of the quality and importance of channel i to the basestation. If the channel quality SI_(w) falls below the threshold Thduring the T_(i) seconds of testing 380, the channel i is removed fromthe base station's assigned channel list 372 and processing returns 368to the loop 302, 308, 306 (FIG. 11).

If the channel quality is not poor 374, T seconds has not passed sincethe last test of channel i 376, or the quality of channel i does notbecome poor during the test period 380, processing returns 368 to theloop 302, 308, 306 (FIG. 11) without removing channel i from the basestation's assigned channel list.

If channel i is not an assigned channel of the base station 364, thebase station can "borrow" channel i from another base station. Ifchannel i is in good quality (S/I≧Th) 382, channel i is not in a locallyheavy loaded period 384, and channel i has been idle for K seconds 386,then channel i may be "borrowed" by the base station 388. Processingthen returns 368 to the loop 302, 308, 306 (FIG. 11). If channel i is ingood quality (S/I≧Th) 382 but channel i is in a locally heavy loadedperiod 384, channel i can be reassigned to the base station if thechannel is in good quality for at least T_(i) seconds--i.e., channel iis monitored for T_(i) seconds 392 and if the co-channel interference,SI_(w), for channel i is greater than or equal to Th during the T_(i)seconds 394, the channel i becomes an assigned channel of the basestation 390. If the channel quality is not good (S/I<Th) 382, or channeli has not idled for K seconds 386 when channel i is not locally heavyloaded 384, or the co-channel interference is measured poor (SI_(w) <Th)394 when channel i is heavy loaded 384, processing will return 368 tothe loop 302, 308, 306 (FIG. 11) without "borrowing" or reassigningchannel i.

The assigned channel list of each base station is maintained in thememory 64 of the base station 21 (FIG. 3). The determination of whethera channel i is in a locally heavy loaded period at the base station 21a(FIG. 2) requires knowledge of the co-channel locations. Therefore, theco-channel location information on each channel i is maintained at thenode 26 or the central control 28 (FIG. 1). Thus, the optimal dynamicchannel allocation method of the alternate embodiment of the presentinvention is not fully distributed because information necessary todetermine the first tier co-channel interference is shared among thebase stations.

By now it should be appreciated that there has been provided anapparatus for channel allocation and a preferred fully distributeddynamic channel method and an alternate optimal dynamic channel methodof operation of the apparatus which combines the benefits of fixedchannel allocation methods and dynamic channel allocation methods and isadaptive to the slowly changing environment while approaching themaximum system capacity with acceptable service quality for anyenvironment.

II. A Second Embodiment of the Handset

FIGS. 15 through 20 show various aspects of an alternate embodiment forthe handset of FIG. 6. Referring to FIG. 15, a channel matrix of thedynamic channel assignment system operates in accordance with thepreferred embodiment of the present invention. The matrix has a numberof radio frequency carriers, F1 through FN, upon which a plurality ofbase stations may communicate with the handset. Each frequency carriesinformation which is time division multiplexed within a predeterminednumber of time slots, TS1 through TSM, each time slot repeats twiceduring a cycle. A time slot pair and a corresponding frequency define achannel for communication between the handset and the base station.During an outbound portion of the cycle, the time slot is forcommunication from the base station to the handset, and during theinbound portion of the cycle, the time slot is for communication fromthe handset to the base station. In the preferred embodiment of the DECTsystem, there are ten frequencies (N=10), each having twenty four timeslot pairs (M=24), and the cycle repeats every 0.01 seconds.

The channels may be reused in differing areas. For example, a first basestation may use TS1 and TS3 of F1 for communication with a first andsecond handset in a first area and a second base station may use TS1 andTSM of F1 for communication with a third and fourth handset in a secondnearby area. Channel reuse in nearby areas (TS1, F1 in this example)allows for more handsets to be accommodated by the communication system.

Referring to TS1 of FN, the generalized structure of the informationwithin a time slot is shown. The information contains a synchronizationportion, 500, which includes bit and frame synchronization signals, anda digital information portion 502 which includes information codewordsformatted with an error detecting code. The error correcting code ispreferably a cyclic redundancy code such as a codeword having a 16 bitCRC which allows for detection of bit errors occurring in thetransmission between the base station and the handset.

The digital information is modulated upon the radio frequency carrierusing two level FM modulation. FIG. 16 shows an idealized eye pattern oftwo level FM modulation operating in accordance with the preferredembodiment of the present invention. The eye pattern represents anoscilloscope display of an accumulation of voltage traces of the asignal demodulated by receiver 174. A symbol communicates one binarybit, thus two distinct levels occur at the center of the symbol, line510. The lowest voltage level represents the logic state of "0", thehighest voltage represents a logic state of "1". Under conditions wherethere is little interference with the carrier frequency by eitherinterfering signals or noise, the voltage levels at the center, line510, of the eye will substantially correspond to the two voltage levelsshown by 0, and 1. However, as the interference combines with thecarrier frequency, the voltages at the center, line 510, of the eyedegrade away from the levels 0 and 1, toward the threshold 514 whichdivides the two levels. Also, the voltage levels may exceed either the 0or 1 voltages. This degradation is shown by four double headed errorsalong line 510. By measuring or sampling the voltage at the center ofthe eye and determining the difference between the sampled voltage and aclosest level of the two levels, an amount of "eye closure", may bedetermined. The amount of eye closure corresponds to a level ofinterference with the carrier frequency by either noise or aninterfering signal, and may be used to assign a value to the informationsignal quality. Alternate embodiments may use other modulationtechniques such as 4 level FM, M-ary FSK, PSK, and QAM without departingfrom the scope of the concept of measurement of "eye closure".

In the first embodiment of Section I, the quality of the channel wasdetermined by the handset in response to the received signal strengthindication (RSSI). RSSI measures the power of the carrier frequencyreceived by the handset. However, in an environment where there ischannel reuse by nearby base stations, the RSSI may not give a reliableindication of the true quality of the channel. This is because the powerof the carrier frequency of a desired base station may combine with thepower of a carrier frequency of a nearby interfering base station. Thecombination results in a higher level RSSI, while resulting in a lowerlevel of quality of the demodulation information due to theinterference.

FIG. 17 shows a flow chart of a first method for determining channelquality. The method operates in microprocessor 178 and may be used bytasks within the microprocessor for determining the quality of achannel. At step 520, the method is invoked. At step 522, the RSSI fromreceiver 174 is measured and the signal of FIG. 16 is sampled and the"eye closure" for each symbol is determined. Step 524 cause the samplingof step 522 to continue for at least a portion of the channel. When thesampling of step 522 is complete, step 524 enables step 526 to determinethe channel quality. The channel quality is analyzed by adding theaverage of the RSSI value and the average of the "eye closure"multiplied by a constant "K".

FIG. 18 shows a flow chart of a second method for determining signalquality. The method operates in microprocessor 178 and may be used bytasks within the microprocessor for determining the quality of achannel. At step 530, the method is invoked. At step 532, the RSSI fromreceiver 174 is measured and the number of bit errors of the digitalinformation signal 502 is determined. Step 534 causes step 532 tocontinue receiving and measuring for the duration of the channel. Whenstep 532 is complete, step 534 enables step 536 to determine the channelquality. The channel quality is determined by adding the average of theRSSI value and the average of bit errors multiplied by a constant "K".

FIGS. 17 and 18 shows that the channel quality signal generated by themethod is a function of analyzing both the carrier frequency power andthe quality of the information modulated upon the carrier frequency. Theflowchart of FIG. 17 analyzes the "eye closure" as a measurement of theinformation quality, and the flowchart of FIG. 18 analyzes the bit errorrate as a measurement of the information quality. The value "K" in steps526 and 536 allows for scaling the information quality relative to thecarrier signal quality. This allows the channel quality measurement tobe adjusted to the expected environment of the handset. For example, inan environment where there is expected to be little channel interferencefrom competing base stations, the value of K may be a small or a deminimis value, thereby making the channel quality predominantly afunction of the carrier frequency power. However, in an environmentwhere the channel interference from other base stations is expected tobe great, the value of K can be increased to a desired level. The valuemay be increased linearly to such a point where the channel quality ispredominantly a function of the information quality, and not carrierfrequency power. By providing the value of "K", a communication systemdesigner has more flexibility in designing the communication system.Thus, what is provided is a handset having an improved channel qualitymeasurement and being capable of accounting for interfering channels.

In a variation of the embodiments of FIGS. 17 and 18, step 524 or 534could limit the sampling of step to only a portion of a channel therebyenabling the receiver 174 to be switched off for the remainder of thechannel. This would both conserve battery power of a battery (not shown)powering the handset and provides for a more rapid measurement ofchannel quality. Alternately, step 524 or 534 could continue the processof steps 522 and 532 for a multiple of occurrences of the channel,thereby providing a more stable time averaged value of channel quality.

FIG. 19 shows a flowchart for reducing power consumption while formaintaining a handset channel list (HCL). Since the handset may have toinitiate or receive a call at any time, a list of good quality channelsmust be continuously maintained by the handset. Since power is consumedfrom the battery powering the handset in maintaining the HCL, it isdesirable to maintain the HCL while minimizing the power consumed by thehandset so that the life of the battery may be extended.

The method of FIG. 19 is initialized to a first channel such as timeslot 1 of F1 of FIG. 15 in step 550. Then in step 552 the receiver 174is switched ON and power is thus consumed from the battery. Then in step554, the channel is received and channel information is decoded andstored for later use in a memory location associated with the channel.The channel information includes a channel ID transmitted by the basestation which indicates the system owner and/or location of the basestation. Also included in the channel information is the base station'sallocated channel list (ACL) of FIG. 5 step 140. Note that priorreferences in the first embodiment to the base station communicates the"PCL" to the handset should more appropriately indicate that the basestation communicates the "ACL" to the handset, although in many cases,the PCL and the ACL may be substantially similar. Then in step 554, thechannel quality is measured according to either FIG. 17, or FIG. 18, orany other method used for determining channel quality (such as RSSIonly). Once the channel quality is determined, the channel isprioritized according to it quality and stored in the HCL contained inmemory 179. In the HCL, the highest quality channel is given the highestpriority and the lowest quality channel is given the lowest priority. Ifall channels are not scanned in step 556, then step 558 initializes forthe next channel and step 554 is repeated. If all channels are scanned,then the receiver 174 is switched OFF at step 560, thereby conservingbattery power. However, the receiver may remain ON if another task inthe microcomputer 178 requires use of the receiver. Then step 562analyzes the channel quality of the channels in the HCL. step 562 usestwo channel quality thresholds, TH1 and TH2, wherein TH2 is greater thanor equal to TH1. If no channel has a quality level above TH1, at step562, then a minimum power conservation delay is set in step 564 which ispreferably 0.01 seconds. However, if at least one channel is valid andhas a channel quality level above TH2, then the battery saving delay isincreased by a predetermined value T at step 566, the value of "T" beinga multiple of 0.01 seconds. Note that a limit on a maximum delay mayalso be included in step 566 which would preferably be one second. Ifthere is at least one channel having a quality between TH1 and TH2 atstep 562, then the delay is left unchanged. Then step 568 delays (withthe receiver OFF) for the amount of time from either step 564 or step566. In alternate embodiments, the value T need not be constant, but maybe a non linear value being a function of the current value of T and/orthe number and/or quality of the scanned channels.

Thus, in a poor channel quality environment, all channels having a valuebelow TH1, the handset scans the channels within a first (minimum) timeinterval and expends a maximum amount of energy scanning for highquality channels to update the HCL. However, if the channel qualityimproves beyond TH2, for reasons including either a reduction in thelocal traffic handled by the communication system, or the handset beingmoved closer to a base station, the scanning rate is adjusted byincreasing the delay established by step 566. Since the delay isincreased, over time less power is expended maintaining the HCL andbattery power is conserved. But, if the handset is again operating in anenvironment where the quality of all channels is less than TH2 butgreater than TH1, the scanning remains unchanged. Finally, if thechannel quality falls below TH1, the delay returns to the minimum, step564, and the maximum amount of energy is expended in scanning for highquality channels.

Further, in the preferred embodiment, after a first full scan of all ofthe channels, additional battery saving (not shown) may be realizedwithin the operation of steps 554, 556 and 558 by battery saving duringthe occurance of channels which been determined to have very lowquality, or the channel ID is invalid because it does not match achannel, system ID or location assigned to the handset. In this way,only high quality valid channels are scanned and power is conservedduring the occurrence of low quality or invalid channels. As the qualityof the scanned high quality channel drops, a full scan may again beperformed.

FIG. 20 shows a flow chart of a handset initiating a call andmaintaining the call during a handover environment. A signal to initiatea call is accepted in step 580. Then in step 582, the handset determineswhich channel has the highest quality on the HCL of FIG. 19 and achannel quality greater than a predetermined handover threshold (HTH)plus a constant delta, step 582. If no channel has a quality greater,then an out of range indication may be sent to the user of the handset.In response to selecting a channel, in step 584, the handset retrievesfrom memory the ACL associated with the channel. The ACL having beenpreviously received in step 554. After receiving the ACL, the handsetthen selects an optimum channel from the ACL for communication with thebase station, step 586. Determination of an optimum channel may beeither by selecting prioritized ACL channel from the base station, or(preferably) selecting a first ACL channel having a weak RSSImeasurement, thereby indicating no interfering sources near the handset,or selecting an ACL channel in response to the handset's own HCL. Eitherof these methods assures a high quality channel connection because, thehandset first selects the highest quality base station. The base stationhaving already sent the handset a list of its determination of thehighest quality available channels. The handset then may select from thebase station's list a channel which it also determines to be a highquality channel.

While the call information is communicated between the base station andthe handset, the handset monitors the channel quality of its connectionwith the base station, step 588. Step 590 then analyzes the channelquality which may include the methods of either FIGS. 16 or 17. As longas the channel quality (CQ) is greater than a predetermined handoverthreshold (HTH) and lower than a measurement threshold (MTH), theprogram continues monitoring the channel quality until the call iscomplete, steps 590, 592 and 596. Note that during this interval thetask of updating the HCL of FIG. 19 concurrently runs. However, if thechannel quality is greater than the measurement threshold, thenadditional power may be conserved during the call by inhibiting thepower consumed in the task of updating the HCL, step 594. If the channelquality falls below the handover threshold for reasons which may includechannel interference by another base station or the handset being movedfarther away from the base station, then step 590 causes the program toreturn to the process of step 582, where another channel (andpotentially another base station) is selected. Note that the handoverprocess of changing from a current channel to a new channel for thehandset occurs in the re-execution of step 586.

Use of the dual threshold in FIG. 20 has several advantages. First,since the handover threshold is lower than the handover threshold plusdelta, once a handset establishes communication with the base station,the signal quality will have to degrade by the predetermined deltabefore a handover is initiated. This allows a handset to communicatelonger with a base station without a handover to another channel. Thedual thresholds aspect reduces the amount of handovers. Providing amethod for reducing the amount of handovers is advantageous because, ifevery handset in the communication system were constantly searching fora channel having the highest quality, there would be a significantamount of handovers. The amount of data communication necessary tocomplete the handovers would load the system down with handover data anddecrease the capacity and profitability of the system.

Furthermore, the value of each of the dual thresholds may be set by thesystem designer, in combination with all other thresholds describedabove. Since the designer balances the base station coverage area withthe amount of handovers in the system, providing both thresholdsfacilitates a better balancing according to the needs of the system. Ifhandsets are allowed to move too far from a base station, either thechannel interference from nearby base stations may be excessive therebyreducing the quality of the call, or the channels must be prohibitedfrom use by nearby base stations, thereby reducing channel reuse andreducing system capacity. These factors are also balanced against adesire to minimize the amount of handovers which also reduce the systemcapacity.

Thus, what is provided is a way to improve the channel qualitymeasurement made by the handset. Also what is provided is a method ofimproving the battery life of the handset while scanning. What is alsoprovided is a method for regulating the amount of handovers occurringwithin the system. What is further provided is a method which allows forestablishing a high quality channel between the base station and thehandset, wherein both the handset and the base station make individualdeterminations on the quality of the channel and the resulting channelis a combination of the individual determinations.

What is claimed is:
 1. A method of determining a quality level of acommunication channel having an information signal modulated upon aradio frequency carrier, the method comprising the steps of:(a)receiving the radio frequency carrier; (b) determining a received signalquality level of the radio frequency carrier; (c) demodulating theinformation signal; (d) determining an information quality level of theinformation signal; (e) analyzing the received signal quality level andthe information quality level comprising:multiplying the informationquality level by a predetermined constant to produce a weightedinformation quality level; and adding the received signal quality levelto the weighted information quality level; and (f) generating a channelquality level signal in response to said step of adding.
 2. The methodof claim 1 wherein the step (b) of determining further includes thesteps of:(b1) measuring a level of radio frequency energy receivedduring said step (a) of receiving; (b2) assigning a value to thereceived signal quality level in proportion to said step (b1) ofmeasuring.
 3. The method according to claim 2 wherein the receivedsignal quality level corresponds to a received signal strengthindicator.
 4. The method according to claim 1 wherein the informationsignal includes a plurality of digital symbols, each of the plurality ofdigital symbols representing at least a first and a second binary state,the first binary state being represented by a first predeterminedvoltage, and the second binary state being represented by a secondpredetermined voltage, wherein said step (d) of determining determinesthe information quality level by:(d1) measuring a symbol voltage of areceived symbol of the plurality of symbols to more closely correspondto the first predetermined voltage than the second predeterminedvoltage; (d2) determining a difference between the symbol voltage andthe first predetermined voltage; and (d3) assigning a value to theinformation quality level in proportion to said determination of saidstep (d2) of determining.
 5. The method according to claim 1 wherein theinformation signal includes digital data comprising at least onecodeword formatted with an error detecting code for detecting up to apredetermined number of errors in the codeword, and said step (d) ofdetermining further includes the steps of:(d3) processing a receivedcodeword and generating an error signal indicative of a number of errorsdetected by the error detecting code; and (d4) assigning a value to theinformation quality level in proportion to the error signal of said step(d3) of processing.
 6. The method according to claim 5 wherein the errordetecting code is a cyclic redundancy code.
 7. The method according toclaim 1 further comprising the steps of:(g) setting the predeterminedconstant to a de minimis value if the radio frequency carrier isexpected to be received with substantially no radio frequencyinterference; and (h) increasing the predetermined constant inproportion to any expected radio frequency interference.
 8. The methodaccording to claim 1 wherein the communication channel is a firstcommunication channel of a plurality of communication channels, and themethod further comprises the steps of:performing said steps (a) through(f) on the first communication channel to produce produce a firstchannel quality level signal; storing the first channel quality levelsignal along with a signal indicative of the first channel in a firstposition in a channel list; performing said steps (a) through (f) on asecond communication channel of the plurality of communication channelsto produce produce a second channel quality level signal; storing thesecond channel quality level signal along with a signal indicative ofthe second channel in a second position in the channel list, wherein thefirst position has a higher priority than the second position if thefirst channel quality level signal is greater than the second channelquality signal, and the first position has a lower priority than thesecond position if the first channel quality level signal is less thanthe second channel quality signal.
 9. A portable handset for determininga quality level of a communication channel having an information signalmodulated upon a radio frequency carrier for communication between thehandset and a base station, the handset comprising:a receiver forreceiving the radio frequency carrier and for generating a receivedsignal strength level indicative of a received signal quality level ofthe radio frequency carrier, said receiver for further demodulating theinformation signal; and a controller for determining an informationquality level of the information signal, analyzing the received signalstrength level and the information quality level by multiplying theinformation quality level by a predetermined constant to produce aweighted information quality level and adding the received signalstrength level to the weighted information quality level, and generatinga channel quality level signal in response thereto.
 10. The portablehandset of claim 9, wherein the controller sets the predeterminedconstant to a de minimis value if the radio frequency carrier isexpected to be received with substantially no radio frequencyinterference, and increases the predetermined constant in proportion toany expected radio frequency interference.
 11. The portable handset ofclaim 9, wherein the controller multiplies an average of the informationquality level by the predetermined constant and adds the weightedinformation quality level to an average of the received signal strengthlevel.
 12. The method of claim 1, wherein the step of multiplyingcomprises multiplying an average of the information quality level by thepredetermined constant to produce the weighted information qualitylevel, and the step of adding comprises adding an average of thereceived signal quality level to the weighted information quality level.